Thermoacoustic vibrations represent a danger for every type of combustion application. They lead to high-amplitude pressure fluctuations, to a limitation in the operating range and they can increase the emissions associated with the combustion. These problems occur particularly in combustion systems with low acoustic damping, such as are often presented by modern gas turbines.
In conventional combustion chambers, the cooling air flowing into the combustion chamber acts to dampen noise and therefore contributes to the damping of thermoacoustic vibrations. In order to achieve low NO.sub.x emissions, an increasing proportion of the air is passed through the burner itself in modern gas turbines and the cooling air flow is reduced. Because of the associated lower level of noise damping, the problems discussed at the beginning correspondingly occur to an increased extent in modern combustion chambers.
One noise-damping possibility consists in the coupling of Helmholtz dampers in the combustion chamber dome or in the region of the cooling air supply. In the case of restricted space relationships, which are typical of modern, compact designs of combustion chambers, however, the accommodation of such dampers can introduce difficulties and is associated with a large measure of design complication.
A further possibility consists in controlling thermoacoustic vibrations by active acoustic excitation. In this procedure, the shear layer which forms in the region of the burner is acoustically excited. A suitable phase lag between the thermoacoustic vibrations and the excitation makes it possible to achieve damping of the combustion chamber vibrations. Such a solution does, however, require the installation of additional elements in the region of the combustion chamber.
It is similarly suitable to modulate the fuel mass flow. In this procedure, fuel is injected into the burner with a phase shift relative to measured signals in the combustion chamber (for example, relative to the pressure) so that additional heat is released at a pressure minimum. This reduces the amplitude of the pressure vibrations.